1. Field of the Invention
The invention relates to an optical semiconductor device having a phototransistor.
2. Related Art
An example of optical semiconductor devices is an OEIC (Optical Electronic Integrated Circuit) device in which a light-receiving element (such as a photodiode) for converting an optical signal to an electric signal, and an active element (such as a transistor element) and a passive element (such as a resistor element and a capacitor element) which form a peripheral circuit are formed on a common substrate. Such an OEIC has been used as various kinds of optical sensor devices, optical pickup devices for optical discs, and the like as a function to convert an optical signal to an electric signal.
Higher light-receiving sensitivity and operation speed have been demanded for OEICs that are used as an optical pickup device. In addition to optical pickup devices for CDs (Compact Discs) using infrared light and for DVDs (Digital Versatile Discs) using red light, optical pickup devices for BDs (Blu-ray Discs) using blue light have come into use recently. Therefore, there are currently three types of optical pickup devices altogether. With improvement in operation speed and multilayer recording of BDs, a higher operation speed and lower noise have been demanded for the OEICs.
Regarding OEICs for BDs, the demand for noise characteristics (S/N (signal-to-noise) ratio) is strict due to the low reflectance of blue light from optical discs. The noise characteristics can be improved by either increasing S (signal) or reducing N (noise). S can be increased by improving light-receiving sensitivity of a light-receiving element. In OEICs using a Si (silicon) photodiode, however, light-receiving sensitivity to blue light has almost reached the theoretical limit. Therefore, an APD (Avalanche Photo-Diode) or phototransistor having an amplifying function in the light-receiving element and providing high light-receiving sensitivity may be mounted in the OEICs for BDs.
A phototransistor is an element structured to amplify an output of a photodiode by a transistor and has been popular as a light-receiving element of an optical sensor which is combined with a light-emitting element such as an LED (Light Emission Diode). In OEICs as well, the amplifying function of the phototransistor can be used in order to improve the light-receiving sensitivity to blue light.
In general, phototransistors are superior to photodiodes in the light-receiving sensitivity because the phototransistors have an amplifying function. However, phototransistors are inferior to photodiodes in the operation speed.
In a conventional example described in Japanese Patent Laid-Open Publication No. 5-235394 (Patent document 1), especially in FIG. 1(b) thereof, a light-receiving surface is formed only by a base of a phototransistor, and an emitter thereof is formed only at an end of the light-receiving surface. When light is incident on the light-receiving surface, carriers generated at the light-receiving surface travel through the base to the emitter. The operation speed of the phototransistor depends on the travel speed of the carriers to the emitter. Since the carriers travel to the emitter by diffusion, the speed is decreased due to a resistance component of the base. The structure of Patent document 1 cannot improve the operation speed due to a long horizontal travel distance of the carriers.
In order to solve this problem, it is known in the art that the operation speed is improved by forming the emitter in the whole surface of the base to reduce the horizontal travel distance of the carriers in the base. For example, such a method is disclosed in FIG. 5 of Japanese Laid-Open Patent Publication No. 9-186355 (Patent document 2) and the like.